RF Fields Applied to Rod Assemblies

It is perhaps worth noting here that, if a quadrupole assembly is used in this all-RF mode, there is no significant mass separation as ions of different mass move through the guide. However, if a DC potential is applied to one pair of rods, the guiding potential changes to that shown in Equation 49.5, in which F is the applied DC potential. 

Therefore, in the RF mode, ions transmitted through the rod guide are subjected to (1) an oscillation in step with the variations of the RF field in the x,y-plane, (2) a drift or guided motion caused by the inhomogeneity of the RF field (x,y-plane), and (3) a forward motion (z-direction) due to any initial velocity of the ions on first entering the rod assembly. The separate motions 

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The equations of motion used to describe the trajectory of an ion in a linear quadmpole (Equation [6.12], Section 6.4.2) are strictly valid only well inside the rod assembly, well removed from the entrance and exit. At each of these ends the ideal quadmpole field (Equation [6.11]) terminates abmptly, but in any real device is affected not only by the RF and DC potentials applied to the rods but also by the potentials applied to nearby ion optical elements (lenses etc.). Moreover, the field lines created by the potentials applied to the rods spill out for some distance outside the theoretical boundaries. These curved fringe fields (Section 6.4.2a) distort the ideal quadmpole field such that the ion motions in the x- and y-directions that are independent of one another in the main quadmpole field (Equation [6.11]) become coupled as a consequence of mixing radial and axial potentials, i.e. the electrical force exerted on an ion in the z-direction can be a function of the time dependent potentials applied in the X- and y-directions (but now curved in three-dimensions), and vice versa. These effects of fringe fields are important in the following discussion. 

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

A hexapole assembly of rods (poles) is built similarly to the quadrupole, but now there are three sets of opposed rods evenly spaced around a central axis. The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode. It is therefore a wide band-pass filter and is used to collimate an ion beam. (Like-charged particles repel each other, and an electrically charged beam will tend to spread apart because of mutual repulsion of ions unless steps are taken to reduce the effect.)... 

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